GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 307 



that this has invariably been the case, and that large areas have 

 invariably been affected by the same movements. When two for- 

 mations have been deposited in two regions during nearly, but 

 not exactly, the same period, we should find in both, from the 

 causes explained in the foregoing paragraphs, the same general 

 succession in the forms of life; but the species would not exactly 

 correspond; for there will have been little more time in the one 

 region than in the other for modification, extinction, and immi- 

 gration. 



I suspect that cases of this nature occur in Europe. Mr. Prest- 

 wich, in his admirable Memoirs on the eocene deposits of Eng- 

 land and France, is able to draw a close general parallelism be- 

 tween the successive stages in the two countries; but when he 

 compares certain stages in England with those in France, al- 

 though he finds in both a curious accordance in the numbers of 

 the species belonging to the same genera, yet the species them- 

 selves differ in a manner very difficult to account for considering 

 the proximity of the two areas, unless, indeed, it be assumed that 

 an isthmus separated two seas inhabited by distinct but contem- 

 poraneous faunas. Lyell has made similar observations on some 

 of the later tertiary formations. Barrande, also, shows that there 

 is a striking general parallelism in the successive Silurian deposits 

 of Bohemia and Scandinavia; nevertheless he finds a surprising 

 amount of difference in the species. If the several formations in 

 these regions have not been deposited during the same exact pe- 

 riods — a formation in one region often corresponding with a blank 

 interval in the other — and if in both regions the species have 

 gone on slowly changing during the accumulation of the several 

 formations and during the long intervals of time between them; 

 in this case the several formations in the two regions could be 

 arranged in the same order, in accordance with the general suc- 

 cession of the forms of life, and the order would falsely appear to 

 be strictly parallel ; nevertheless the species would not be all the 

 same in the apparently corresponding stages in the two regions. 



ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, 

 AND TO LIVING FORMS 



Let us now look to the mutual affinities of extinct and living 

 species. All fall into a few grand classes; and this fact is at once 

 explained on the principle of descent. The more ancient any form 

 is, the more, as a general rule, it differs from living forms. But, 

 as Buckland long ago remarked, extinct species can all be classed 

 either in still existing groups, or between them. That the extinct 



